Preloaded spring, method and apparatus for forming same

ABSTRACT

A closely wound, coil type tension spring of one hand having an initial tension exceeding that obtainable by prior spring forming techniques is formed by reverse winding an open-wound compression spring of the opposite hand. Alternative methods and apparatus for forming the tension spring are disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 814,234, filedDec. 30, 1985. now U.S. Pat No. 4,719,683.

BACKGROUND OF THE INVENTION

It is known that during a tension spring forming operation, a twist maybe introduced in the spring wire, such that adjacent coils tend to pressagainst one another and resist subsequent deflection of the spring. Aspring of this type is considered to have an "initial tension" or a"pre-load" (Pi), which must be overcome by a tension force applied tothe spring before the spring coils will open or separate The amount ofinitial tension present in a tension spring, which is formed of a givenspring material, wire diameter "d" and mean spring diameter "D", may becontrolled within limits depending upon the degree of bending of thewire incidental to the coiling of such wire to form the spring, and thedegree to which the formed spring is subsequently stressed relieved by aheat treating operation.

Initial tension or preload is an important factor in obtaining a desiredspring load at a desired spring length and in changing spring loads fora given spring design without changing wire size, spring diameter, thenumber of coils and the slope of the spring gradient or spring constant.As by way of example, the full extension for a given tension spring,i.e., that is the extension required to be applied to extend a spring toits highest load without inducing permanent deformation of the spring,may be increased or decreased by changing the initial preload placed inthe spring as it is formed. It is also oftentimes desirable to design atension spring having the lowest spring gradient possible over a certainrange of extension, and one way this has been accomplished is toinitially wind the spring with maximum pretension. This produces aspring in which substantial spring force is available for use beforeextension of the spring is initiated and thus the spring may be formedwith more coils, while still being fitted within a given space.

An empirical formula developed by Hunter Spring of Hatfield, Pa., adivision of Ametek, Inc., from results from springs of various materialsand proportions indicates that the greatest amount of initial tension,which can be obtained in a conventionally formed close-wound tensionspring, after normal stress relieving, is ##EQU1## wherein "Sy" is themaximum apparent elastic limit for common spring materials appearing ina publication entitled Spring Design Data, copyright 1964 Ametek, Inc.The maximum apparent elastic limit appears to substantially exceed theactual "elastic limit" of such spring materials, due to certainbeneficial internal stresses produced in the wire, during forming of thespring. The disclosure of this publication is incorporated by referenceherein.

SUMMARY OF THE INVENTION

The present invention relates to the manufacture of tension springs andmore particularly to tension springs provided with a higher range ofmaximum initial tension or preloading conditions than previouslyobtainable.

In accordance with the present invention, a preloaded, close woundtension spring of one hand is formed by reverse winding a conventionallyformed open-wound compression spring of opposite hand. Preferable,reverse winding operation is performed under conditions, wherein no orminimum plastic yielding of the spring wire occurs, since reduced valuesof maximum initial tension in the resulting tension spring willotherwise result. Reverse winding under these conditions may be employedto produce a tension spring having substantially the same mean diameteras the compression spring from which it is formed. However, theinvention is not so limited, since it is contemplated that tensionsprings having mean diameters less than or exceeding those of thecompression springs from which same are formed may have utility in thatsame may still possess higher initial tensions than conventionallyformed tension springs of similar spring material, size and wirediameter.

Further, in accordance with the present invention, it is contemplatedthat tension springs having improved preload properties may be formed byeither reverse winding a compression spring outwardly or inwardly uponitself. As an incident to a reverse winding operation, the spring coilsmay be simply temporarily enlarged or reduced in size by a factoressentially equal to twice the spring wire diameter of the compressionspring. However, compression springs having smaller values of the ratioD/d may be successfully rewound without yielding by first decreasing orincreasing the diameters of the coils of compression springs prior toperforming outwardly and inwardly directed rewinding operations,respectively, thereon.

DETAILED DRAWINGS

FIG. 1 is a side elevational view of a spring rewinding apparatus usedin the practice of the present invention;

FIG. 2 is a view taken generally along the line 2--2 in FIG. 1;

FIG. 3 is a sectional view taken generally along the line 3--3 in FIG.2;

FIG. 4 is a sectional view similar to FIG. 2, but showing an alternativeform of the present invention;

FIG. 5 is a sectional view taken generally along the line 5--5 in FIG.4;

FIG. 6 is a sectional view illustrating second alternative form of thepresent invention;

FIG. 7 is a fragmentary perspective view of the left hand arbor shown inFIG. 6;

FIG. 8 is a fragmentary perspective view of the right hand arbor shownin FIG. 6.

FIG. 9 is a sectional view illustrating a third alternative form of thepresent invention;

FIG. 10 is a fragmentary view taken generally along the line 10--10 inFIG. 9;

FIG. 11 is a fragmentary perspective view of the arbor constructionshown in FIG. 9; and

FIGS. 12a and 12b are graphical illustrations plotting the ratio ofinitial tension factor against the ratio of mean spring diameter to wirediameter.

DETAILED DESCRIPTION

In accordance with the present invention, an open-wound compressionspring 10, which is shown in its as formed configuration in FIG. 1, issubjected to a reverse winding operation to create a preloaded orinitially tensioned, close-wound tension spring 10' shown in part inFIG. 3 in the act of its formation. The characteristics desired to beimparted to tension spring 10' will determine the type of springmaterial employed in forming spring 10; its Index, that is, the ratio ofits mean diameter "D" to its wire diameter "d"; its number of coils 12;its pitch "P" or the center to center distance between adjacent coils;and whether the compression spring is heat treated for stress relievingpurposes prior to the reverse winding operation.

A presently preferred apparatus for use in forming spring 10' is shownin the drawings as including a winding head 14 and a pusher device 16.Head 14 is defined by a stationary frame 18 having a centrally locatedblind bore 20, which opens through its rear surface and is sized toreceive a bearing sleeve 20a for rotatably supporting one end of a driveshaft 22 fitted with a drive gear 22a. Frame 12 is formed with threethrough bores 24, only one of which is shown in FIG. 3, which areuniformly annularly spaced apart and arranged equidistant from the axisof bore 20. Bores 24 are fitted with bearing sleeves 24a, only one ofwhich is shown in FIG. 3, for rotatably supporting first, second andthird shafts 26, 28, and 30 having gears 26a, 28a and 30a and drivepulley elements 26b, 28b and 30b fixed to their rear and front ends,respectively. As will be apparent from viewing FIG. 2, gears 26a, 28aand 30a are arranged in driven engagement with drive gear 22a, such thatupon rotation of drive shaft 22, pulley elements 26b, 28b and 30b aredriven for rotation in a desired direction. The direction in whichspring 10 is wound determines the relative axial placement of the pulleyelements and the direction in which same are driven.

By making reference to FIGS. 2 and 3, it will be understood that pulleyelements 26b, 28b and 30b are radially positioned and sized such thattheir radially facing, annular drive surfaces 26c, 28c and 30c define aneffective diameter sufficient for frictionally engaging with theradially inner surfaces of coils 12 preferably without effectingdeformation thereof By reference to FIG. 3, it will also be understoodthat drive surfaces 26c, 28c and 30c are axially spaced one from anothersuch that they lie along a helix determined by the helix defined bycoils 12 when spring 10 is compressed by pusher device 16 sufficientlyto bring its adjacent coils into contact. When spring 10 is right handwound, as shown in the drawings, pulley elements 26b, 28b and 30bconstitute lead, intermediate and trailing pulley elements and arearranged in the uniform axially stepped relationship shown in FIGS. 1and 3, wherein pulley element 26b is spaced furthest from frame 18. Forthe illustrated construction, the pulley elements would be driven in aclockwise direction, as viewed in FIG. 2.

Frame 18 is also provided with an additional or fourth through bore 32,which is arranged intermediate the shafts 26 and 30 and fitted with abearing sleeve 32a for rotatably supporting a stub shaft 34 having guidemeans in the form of a guide pulley element 34b mounted on its frontend. Shaft 34 is radially positioned and pulley element 34b sized, suchthat its radial guide surface 34c is disposed radially outwardly ofsurfaces 26c, 28c and 30c through a distance preferably essentiallyequal to wire diameter "d". Surface 34c preferably is arranged in anaxial position in which it lies along the helix bisecting surfaces 26c,28c and 30c, and thus is positioned axially intermediate surface 20c andframe 18, but it is presently anticipated that surface 34c may bealigned or even positioned slightly in front of surface 30c.

By again referring to FIGS. 2 and 3, it will be noted that frame 18 isalso fitted with thrust race 36, which includes an annular thrust ring38 supported for free rotational movement about an axis arranged inalignment with the axis of drive shaft 22. The radial dimension ofthrust ring 38 is such that coils 12 engage with such ring after samehave passed over guide pulley element 34b. The axial distance or spacingbetween thrust ring 38 and the center of the portion of drive surface26c engaged with spring 10 is equal to and preferably slightly less thanwire diameter "d".

Pusher device 16 includes a support shaft 40 arranged in co-axialalignment with drive shaft 22 and a disc shaped pusher plate or head 42.Pusher plate 42 has a cylindrically shaped, rear guide portion 44 whosediameter essentially corresponds to the effective diameter defined bydrive surfaces 26c, 28c and 30c, that is, the inside diameter of spring10 defined as "D" minus "d"; and a forwardly inclined, frustoconicallyshaped, front guide portion 46 having an outer diameter essentiallyequal the mean spring diameter "D" and inner or smaller diameteressentially equal to inside diameter of spring 10. Pusher device 16 maybe supported for free rotational movement, but it is preferably drivenfor rotation in the direction of rotation of pulley elements 26, 28 and30. Any suitable means may be provided for driving shafts 22 and 40.

In accordance with the present invention, the leading coil 12a of spring10 is threaded successively about pulley elements 26b, 28b, 30b and 34buntil its free end 50 engages with thrust ring 38, whereafter pusherdevice guide portion 44 is slid within the trailing coil 12b of spring10 and pusher device 16 moved towards winding head 14 in order tocompress spring 10 sufficiently to place its adjacent coils 12 insurface-to-surface engagement. Thereafter, shaft 22 and preferably shaft40 are driven, and shaft 40 biased by suitable means, not shown, formovement progressively towards winding head 18 to maintain the coils ofspring 10 in a closed state, until such spring is wholly reverse woundto form spring 10'.

The reverse winding operation will be more easily understood by firstnoting that guide pulley element 34b serves to initiate spreading orenlargement of the diameter of the coils 12 of spring 10 as same passthereover; and that thrust ring 38 serves to limit the extent of axialmovement of the coils towards the right, as viewed in FIGS. 1 and 3.Thus, as free end 50 and successive portions of spring 10 are fed overguide pulley element 34b and into engagement with thrust ring 38 bydrive pulley elements 26b, 28b and 30b, such free end and such portionsare forced to move radially outwardly and to ride or slide over theouter surface of an adjacent coil trained about pulley elements 26b, 28band 30b for subsequent movement in a direction away from the thrustring, as best shown in FIG. 3. As an incident to such movement, the wireforming spring 10 is twisted to produce new coils 12' comprising tensionspring 10', which are oppositely wound, e.g., coils 12' have a left handwind for the case where the coils 12 of spring 10 had a right hand wind.As reverse winding continues, the free end 50 will eventually ride offof trailing coil 12b and into engagement with frusto-conical portion 46,which permits the new leading coil 12a'to resiliently contract until itassumes some predetermined final mean diameter.

In the presently preferred method of forming a tension spring, thecompression spring would be heat treated to remove undesired residualstresses, which may interfere with the reverse winding operation, andits characteristics chosen to insure that the wire will be twistedwithout substantially yielding in order to provide a close wound tensionspring having a high preload and a mean diameter essentially equal tothe original spring mean diameter "D".

Reference is now made to FIGS. 4 and 5, which illustrate an alternateform of the present invention, wherein an open-wound compression springof one hand is wound inwardly upon itself to form a close-wound tensionspring of the opposite hand. In that the apparatus of this secondembodiment is quite similar to that previously described with referenceto FIGS. 1-3, primed numerals will be employed to designate similarelements thereof. Thus, in this apparatus, winding head 14' supports oneend of a drive shaft 22' having a gear 22a' arranged to mesh with gears26a', 28a' and 30a' connected to drive pulley elements 26b', 28b' and30b' whose drive surfaces 26c', 28c' and 30c' are arranged tofrictionally engage with the radially outer surfaces of coils 12preferably without effecting deformation thereof. Drive surfaces 26c',28c' and 30c' define lead, intermediate and trailing surfaces, whenspring 10 is right hand wound as shown in FIGS. 4 and 5, and lie along ahelix defined by coils 12 when the compression spring is axiallycompressed by pusher device 16' sufficiently to bring such coils intocontact with each other. Guide pulley element 34b' may be arrangedessentially equidistant from pulley elements 26b' and 30b' in a mannersimilar to the corresponding pulley elements of the first embodiment ofthe invention, but it is preferably arranged relatively closer to pulleyelement 26b', as shown in FIG. 4, so as to reduce the bending stressapplied to coils 12 in passing between pulley elements 30b' and 34b'.

As will be apparent from viewing FIGS. 4 and 5, thrust race 36' isarranged radially inwardly of the drive pulley elements, so as toposition thrust ring 38' for engagement with coils 12 as same pass overguide pulley element 34b'.

Pusher device 16' is shown in FIG. 5 as including a support shaft 40'serving to support a pusher head 42', having an inner diametercorresponding essentially to the outside diameter of spring 10 and afrusto-conically shaped, front guide portion 46'. Guide portion 46' hasa minimum inside diameter corresponding essentially to inner diameter ofcompression spring 10 and a maximum inside diameter essentially equal tothe outside diameter of such spring.

In operation of the embodiment shown in FIGS. 4 and 5, after the leadingcoil of compression spring 10 is threaded over pulley elements26b'28b'30b' and 34b' and its coils are axially compressed by pusherdevice 16', reverse winding is effected by driving free end 50 of theleading spring coil and successive portions of the spring intoengagement with thrust ring 38', which serves to force successive coilsto move radially inwardly and to ride or slide against the innersurfaces of an adjacent coil 12 trained about the pulley elements. As anincident to this deformation of coils 12, new coils 12' of opposite windcomprising spring 10' are formed. As forming of spring 10' progresses,coils 12' ride off or out of spring 10 and slide outwardly alongfrusto-conical portion 46' until they resiliently expand to assume amean diameter corresponding essentially to the mean diameter of spring10. As in the case of the first embodiment of the invention, acompression spring to be inwardly wound upon itself is preferablyinitially heat treated to remove undesired localized stresses, and itscharacteristics chosen to permit rewinding to occur without substantialyielding of the spring material. Preferably, yielding is restricted inorder to provide tension spring 10' with a mean diameter essentiallyequal to that of spring 10 and maximize the initial preload imparted tothe tension spring.

FIGS. 6-8 illustrate a further embodiment of the invention, wherein anopen-wound compression spring 10 of one hand is subjected to radialcompression prior to being wound reverse wound outwardly upon itself todefine a tension spring 10' of the opposite hand. In this embodiment ofthe invention, 51 designates a stationary housing formed with a throughopening 52 having an inlet end 52a of a diameter essentially equal to orgreater than the as-formed outer diameter of a compression spring 10, anoutlet end 52b of a diameter less than such as-formed outer diameter,and an intermediate portion 52c of essentially frusto-conicalconfiguration extending between ends 52a and 52b. This embodiment alsofeatures the use of a first arbor 54 and a second arbor 56, which aresupported for rotation in a common direction determined by the directionof wind of spring 10 and for axial displacements, as indicated by arrows58 and 60, respectively.

First arbor 54 has a diameter essentially equal to and preferably lessthan the diameter of outlet end 52b and is formed adjacent its insertedor right hand end, as viewed in FIG. 6, with a generally cylindrical,reduced diameter guide extension 54a serving to define a slot 54b sizedto receive a tong 62 carried by trailing spring coil 12b, for purposesof rotatably coupling the first arbor to the trailing coil. The diameterof guide extension 54a is equal to and preferably slightly less than thereduced internal diameter of the coils of spring 10, as same passthrough outlet end 52b. Arbor 54 is suitably supported for rotation in adirection opposite to the direction in which spring 10 is wound forpurposes of "screwing" the spring 10 into intermediate portion 52c inorder to progressively radially compress or reduce the diameter of itscoils as they approach outlet end 52b, and for translation towards theright, as viewed in FIG. 6, in order to force the reduced diameter coilsoutwardly through the outlet end. The diameter of outlet end 52b may beequal to or slightly greater than the internal diameter of spring 10depending on the degree to which it is desired to compress such spring.Preferably, the outlet end would be sized to compress spring 10 by aboutone wire diameter "d".

Second arbor 56 is suitably supported for rotation and displacement inaxial alignment with outlet end 52b and is formed with an inverting endportion 56a arranged in a facing relation with such outlet end. Endportion 56a is best shown in FIG. 8 as being defined by an annular ringportion 56b, a central recessed portion 56c arranged parallel to thering portion and a ramp or camming portion 56d, which is disposedconcentrically inwardly of ring portion 56b and progressively increasesin axial thickness or height relative to the surface of central recessedportion 56c in a direction opposite to the direction of rotation of thesecond arbor. The inner diameter of ring portion 56b preferablycorresponds essentially to the diameter of outlet end 52b plus two wirediameters "d", and the ring portion is preferably axially spaced fromcentral recessed portion 56c through a distance essentially equal to orslightly greater than wire diameter "d". Ramp portion 56d has an innerdiameter essentially corresponding to the diameter of outlet end 52b anda maximum height, as measured relative to central recessed portion 56c,of essentially one half of wire diameter "d".

In operation of the embodiment of the invention depicted in FIG. 6,second arbor 56 is initially positioned to place ring portion 56b inclose proximity to or in actual surface engagement with forming member51 concentrically of outlet end 52b. Spring 10 is then inserted intothrough opening 52 and first arbor 54 is rotatably coupled to trailingcoil 12b. First arbor 54 is then rotated and moved inwardly of opening52 for purposes of progressively compressing spring 10 and forcing orfeeding its leading end outwardly through outlet end 52b. As the leadingcoil of spring 10 exits through outlet end 52b into engagement withcentral recessed portion 52c, it tends to resiliently expand to itsinitial or as-formed diameter with its leading end arranged forengagement with ramp 56d. As the leading coil expands and the coil nextthereto attempts to enter end portion 56a, ramp 56d tends to force theleading coil to move towards outlet end 52b with the result that theleading coil is deformed to define coil 12a' and forced to slideoutwardly over its next adjacent coil to initiate forming of tensionspring 10'. Thereafter, movement of second arbor 56 to the right, asviewed in FIG. 6, is initiated while such arbor is simultaneouslyrotated in the direction indicated by arrow 58. The speed of movement ofarbor 56 is correlated with the speed at which coils 12 exit throughoutlet end 52b to ensure that such coils are maintained in axialengagement with one another. As the reverse winding operation continues,leading coil 12a' rides off of trailing coil 12b and onto first arbor54; the first arbor continuing to axially converge towards end portion56a until such trailing coil is placed within such end portion tocomplete the rewinding operation. Rotation of arbor 54 can be relativelyslow since its function is to prevent the spring from binding in thebore. Rotation of arbor 56 must be faster and depends on the rate ofemergence of the spring from the bore. Arbor 56 must rotate once forevery rewound spring turn and move in direction 60 by "d" for every turnto keep the rewound spring confined between block 50 and itself. Arbor54 moves in direction 60 by the amount required to keep spring 10compressed coil to coil. Therefore, arbor 54 moves right at slightlymore than twice the rate of arbor 56.

A further embodiment of the invention is shown in FIG. 9, wherein 70designates a forming member having a cylindrically shaped guide portionor bore opening 72 having a diameter greater than the initial oras-formed outer diameter of a compression spring 10 to be rewound, andan outlet portion 72a having a minimum diameter less than that of theguide portion by an amount essentially corresponding to twice the wirediameter "d" of the spring. Guide portion may have a diameter exceedingthe as-formed outer diameter of spring 10 by upwards of two wirediameters, but about one wire diameter is preferred. Outlet portion 72acooperates with guide portion 72 to define an annular abutment surface72b, which is inclined and arranged to face towards the opposite orinlet end 72a of the guide portion. A portion of forming member 70 iscut away to form a lot 74 shown in FIGS. 9 and 10, which communicateswith guide portion 72 and forms a constraining or second abutmentsurface 74a to be engaged by the free end of first coil 12a, when spring10 is fully inserted into forming member 70. Surfaces 74a cooperate withat least a first arbor 76 for purposes of expanding coils 12 of spring10 into engagement with guide portion 72 and then effecting reversewinding of the spring in the manner to be described. To this end, arbor76 is sized to be slidably and rotatably received within guide portion72 and provided with an inverting end portion 76a having a recessedplanar surface 76b from which upstands a ramp or camming portion 76chaving a spiral shaped radially inner surface 76d. Ramp 76c has auniform axial length or height above surface 76b corresponding to aboutone wire diameter "d", and a radial dimension varying from essentiallyzero at a thin or knife edge end 76e to about one wire diameter "d"adjacent an opposite or enlarged end 76f. Preferably, arbor 76 is formedwith an axially extending bore opening 76g for rotatably and slidablyreceiving a second arbor 78 having a diameter equal to or slightly lessthan the diameter of guide portion 72 less about four wire diameters.The leading end of second arbor 78 is provided with a slot 78a sized toengage a tong 80 defined by the trailing coil of spring 10.

In operation of the embodiment shown in FIGS. 9-11, spring 10 ispositioned with guide portion 72 with its leading coil engaged withsurfaces 72b and 42a. Thereafter, arbor 76 is inserted within guideportion 72, advanced and rotated opposite the direction in which spring10 is wound for purposes axially collapsing spring 10 to essentially itssolid height, as the trailing end of the spring is turned sufficientlyto expand all of coils 12 outwardly into engagement with the guideportion. The trailing coil of the compression spring is rotatablycoupled with arbor 76 due to frictional engagement of such coil withramp inner surface 76d.

When further rotation of the trailing end of the spring is prevented byits engagement with the guide portion 72, it is forced along ramp innersurface 76d and thus reduced in diameter to be able to pass through theexpanded spring 10. Further rotation and advancing of arbor 76 causessuccessive turns to be reduced in diameter. These displace previouslyreduced turns and the spring is thus reverse wound through its inside.The turns inside wish to expand and bind against the turns outside sincemore than one inside turn is produced for every outside turn reduced.Resultant binding is prevented by engaging tong 80 with arbor 78 androtating this arbor in a direction to reduce the diameter of the reversewound portion, i.e., in a direction opposite to that of arbor 76 and ata speed higher by the ratio of the mean outside and inside diameters.

FIGS. 12a and 12b compare initial tension factors obtainable withconventionally wound tension springs and reverse wound tension springsof the resent invention for a relatively low strength spring material,such as phosphorus bronze and a relatively high strength springmaterial, such as music wire, respectively. The curves approximatelyrepresent a situation where the initial tension factor is equal to theratio Pi/P2, where P2 corresponds essentially to the ultimate load of aspring achievable under use conditions of about 50,000 cycles and for aratio of (P2-P1)/P2 of about 0.5. P1 is the load applied to a springwhen installed. P1 will equal Pi where a spring has all its coilstouching when installed for use purposes and will be greater than Piwhere the spring is installed in a slightly extended condition. For areverse wound spring, the theoretical upper limit of possible preloadlies at the point where the combined stresses in torsion and bendingencountered during a reverse winding operation are just below theapparent yield strength of the spring material. However, preloading tothis extent does not permit substantial further stretching of thereverse wound spring during use without yielding and results in lowcyclic life. Thus, a preload level should be chosen, such as that at thestress ratio at which the reverse wound spring is intended to operatewill provide for an acceptable cyclic life. The upper limit of theenvelope of the preload curve illustrated in FIGS. 12a and 12b is notthe theoretically obtainable limit for a reverse wound tension spring,which would correspond to an initial tension factor of almost 1.0.Rather, the upper limits depicted in the drawings is meant toapproximately illustrate a serviceable upper limit, which allows for aminimum useful measure of extension of a spring during use. The lowerlimit of possible preload is taken as the preload resulting from reversewinding a compression spring, which is initially wound such that itsadjacent coils just touch one another. It may be helpful to considerthat for any given value of the ratio D/d, assuming the same springmaterial, wire diameter and wire configuration, that the value of theinitial tension factor for a reverse wound spring will increase withinthe bounds of the envelopes depicted in FIGS. 12a and 12b at a rateproportional to the pitch or the center-to-center distance betweenadjacent coils of the compression spring from which it is formed.

Reverse winding can be achieved without yielding for ratios of D aboveapproximately 14 and 12 for the cases of phosphorus bronze and musicwire spring materials shown in FIGS. 12a and 12b, respectively. Forlower ratios of D/d, progressive yielding of spring material will occurresulting in dimensional changes of the reverse wound spring relative tothe compression spring from which it is formed. This lower limit for theratio D/d for a given spring is determined by the permissible bendingstress and elastic modulus of the spring material and thecross-sectional configuration of the spring wire, which, for purposes ofFIGS. 12a and 12b, is assumed to be round.

By viewing FIGS. 12a and 12b, it will be appreciated that reverse woundsprings formed in accordance with the present invention may be providedwith a preload, which lies within a relatively large envelope or rangeof preloading conditions, wherein the minimum preload condition isgreater than the maximum preload condition obtainable for conventionallywound tension spring except for tension springs formed of relativelyhigh strength spring materials lying within the upper normal range ofthe ratio of D/d.

A plain or single direction reverse winding operation achieved by theapparatus of FIGS. 1-5 is contemplated for use with compression springshaving relatively large values of the ratio D/d. As the value of theratio D/d decreases, it is preferred to reverse wind inwardly throughthe center of a compression spring by use of the apparatus depicted inFIGS. 4 and 5, since the apparent elastic limit of the spring materialis higher when the coil diameter is decreased, during a reverse windingoperation. To achieve reverse winding at relatively low values of theratio D/d, it is required to both expand/contract or contract/expand thecoils of a compression spring to their limit of expansion, as the coilspass over one another, since this permits a greater change of coildiameter than that obtainable by a plain reverse winding operationwithout encountering a yielding condition. Of the alternativeembodiments of the invention depicted in FIGS. 6-8, the embodiment ofFIGS. 9-11 is preferred to perform this dual direction windingoperation.

The presently disclosed methods of forming a tension spring may bedeparted from in certain instances. As by way of example, there may beadvantages in causing yielding of the coils of the tension springincident to formation thereof, so as to create a tension spring having adiameter differing from that of the compression spring from which it isformed. However, such yielding is expected to result in a reduction inthe amount of preload which might otherwise be obtainable. Additionally,it is contemplated that a tension spring may be formed by reversewinding a compression spring coincident with the formation of suchcompression spring; that is the reverse winding of each coil of suchcompression spring immediately after the formation thereof.

Further, while several types of apparatus for forming a tension springby reverse winding a compression spring outwardly or inwardly uponitself has been disclosed, several additional constructions arecontemplated. In this respect, it is contemplated that the drive pulleyelements of the first embodiment of the invention may be replaced by ascrew-like, unitary drive conveyor formed with a helical drive groove onits outer surface. In a like manner, it is contemplated that the drivepulley elements of the second embodiment of the invention may bereplaced by a generally cylindrical drive device having a helical drivegroove on its inner surface.

The expression "wound upon itself" is used broadly to include situationswherein the coils of the tension spring actually ride on or slide alongthe coils of the compression spring from which it is formed, as well asthe situation in which the coils of the tension spring are supported bya suitable guide arranged radially intermediate the tension andcompression springs, during the reverse winding operation. Theexpression "reverse winding a compression spring" is used broadly toinclude situations, wherein the coils of a compression spring arereverse wound one at a time, regardless of whether the compressionspring is operated upon after or during formation thereof.

I claim:
 1. A preloaded, close-wound tension spring of one hand formedby a method including the steps of changing the radial dimension of acompression spring of the opposite hand and then reverse wining saidcompression spring upon itself in a direction opposite to the directionin which said radial dimension is changed, characterized in that saidtension spring has no or comparatively less yield than a tension springsubject to yielding as a result of being formed by reverse winding saidcompression spring upon itself without first changing the radialdimension thereof in a direction opposite to the direction in whichreverse winding occurs.
 2. A tension spring according to claim 1,wherein the step of changing said radial dimension involves radiallyexpanding said compression spring.
 3. A tension spring according toclaim 2, wherein the step of changing said radial dimension involvesradially contracting said compression spring.
 4. A preloaded,close-wound tension spring of one hand formed by a method including thesteps of changing the radial dimension of a compression spring of theopposite hand and then reverse winding said compression spring uponitself in a direction opposite to the direction in which said radialdimension is changed, characterize in that for compression springs ofgiven mean diameter and composition, said tension spring may be formedwith a given degree of yield from a compression spring having a greaterwire diameter than can a tension spring having said given degree ofyield and formed by a method of reverse winding said compression springupon itself without first changing the radial dimension thereof in adirection opposite to the direction in which reverse winding occurs. 5.A preloaded, close-wound tension spring of one hand formed by a methodincluding the steps of radially expanding a compression spring of theopposite hand and then reverse winding said compression spring radiallyinwardly upon itself, characterized in that said tension spring has aD/d ratio less than the D/d ratio of a tension spring formed by reversewinding said compression spring inwardly upon itself without firstradially expanding said compression spring, for any given degree ofyield of the material from which said compression spring is formed,wherein D and d are the mean diameter and wire diameter of saidcompression spring, respectively.
 6. A tension spring according to claim5, wherein said D/d ratio is equal to or less than
 14. 7. A tensionspring according to claim 5, wherein there is essentially no yielding ofsaid material.
 8. A tension spring according to claim 5, wherein saidmethod includes the additional step of stress relieving said compressionspring prior to radial expansion thereof.
 9. A preloaded, close-woundtension spring of one hand formed by a method including the steps ofradially contracting a compression spring of the opposite hand and thenreverse winding said compression spring radially outwardly upwardlyitself, characterized in that said tension spring has a D/d less thanthe D/d ratio of a tension spring formed by reverse winding saidcompression spring outwardly upon itself without first without firstradially contacting said compression spring, for any given degree ofyield of the material from which said compression spring is formed,wherein D and d are the mean diameter and wire diameter of saidcompression spring, respectively.
 10. A tension spring according toclaim 9, wherein said D/d ratio is equal to or less than
 14. 11. Atension spring according to claim 9, wherein there is essentially noyielding of said material.
 12. A tension spring according to claim 9,wherein said method includes the additional step of stress relievingsaid compression spring prior to radial contraction thereof.